NL100: Second barrel situation

[Aquadougs]

Villain is unknown

Party Poker, $0.50/$1 NL Hold'em Cash Game, 6 Players
LeggoPoker.com - Hand History Converter

SB: $177
BB: $129.63
UTG: $116.50
MP: $103.40
CO: $79.40
Hero (BTN): $100.40

Pre-Flop: XX dealt to Hero (BTN)
3 folds, <font color="red">Hero raises to $4</font>, SB folds, BB calls $3

Flop: ($8.50) 6[img]/images/graemlins/heart.gif[/img] 3[img]/images/graemlins/club.gif[/img] 3[img]/images/graemlins/heart.gif[/img] (2 Players)
BB checks, <font color="red">Hero bets $6</font>, BB calls $6

Turn: ($20.50) 8[img]/images/graemlins/spade.gif[/img] (2 Players)
BB checks, <font color="red">Hero?


What should I bet with here?